The invention relates to a device for carrying out measurements on a thin flexible tape material, which device comprises at least one pair of guide elements which are spaced from each other and have guide surfaces facing each other which extend substantially parallel to each other and between which a guide channel for the tape material is formed. Supply ducts terminate in the guide surfaces, for supplying to both sides of a tape material in the guide channel a pressurized gaseous medium by means of which the tape material can be guided in the longitudinal direction in the guide channel along a path which is substantially parallel to and spaced from the guide surfaces. In each of these guide surfaces a termination of at least one measuring duct is situated, the measuring-duct termination or each measuring-duct termination in one guide surface being arranged at least substantially directly opposite the measuring-duct termination or an associated one of the measuring-duct terminations in the other guide surface and forming therewith a pair of measuring-duct terminations. The measuring ducts communicate with at least one pressure transducer.
A device of this type is known from British Patent Specification No. 1,461,248. In this known device the web material is guided between two guide surfaces, the medium pressure on each side being determined continuously. Depending on the distance of the material from the guide surfaces pressure variations occur on both sides of the material, which variations are converted into electrical quantities, which are added to form a sum quantity from which the local thickness of the web material is derived continuously. Such a method is suitable for thickness measurements only, because the measuring means are adapted for determining sum quantities only. Moreover the flat shape of the guide surfaces does not permit the use of measurements of a different type.